In device fabrication, insulating, semiconducting, and conducting layers are formed on a substrate. The layers are patterned to create features and spaces to form devices, such as transistors, capacitors, and resistors. These devices are then interconnected to achieve a desired electrical function.
In some applications, trenches or deep trenches are created in the substrate to form, for example, trench capacitors. Creation of the trenches is achieved by an anisotropic etch such as reactive ion etch (RIE). A hard mask layer is provided and patterned to serve as a RIE mask. Typically, additional layers such as a pad nitride and a pad oxide are provided underneath the hard mask layer. The nitride layer serves as a polish stop layer for subsequent processes and the pad oxide layer promotes adhesion and to reduce stress between the silicon substrate and the pad nitride layer. As such, the hard mask layer needs to be dense enough to withstand the bombardment of ions during the RIE. Additionally, the mask layer should have a substantially higher etch rate than the polish layer, enabling it to be removed without removing the other pad layers.
Conventionally, TEOS oxide is used as a hard mask layer. TEOS is sufficiently dense to withstand the RIE. However, TEOS cannot be etched selectively to oxide. This can cause problems during removal of the TEOS hard mask. For example, removal of the TEOS hard mask also etches the pad oxide underneath the pad nitride, resulting in the separation of the pad nitride from the substrate.
As evidenced from the above discussion, it is desirable to provide a pad stack with a hard mask layer that can be removed selective to oxide.